Numbers behind electric vehicles don’t add up

The Mercedes SLS AMG E-Cell I tested has a battery rated at 48 kilowatt-hours. It weighs 450 kilograms.

PHOTO: Stan Honda, AFP/Getty Images

Gasoline packs a punch; batteries not so much

By David Booth, National Post

Originally published: July 14, 2010

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It’s all in the numbers. Whether the source is gasoline, a newfangled hydrogen fuel cell or lithium-ion batteries (the media’s current darling), it takes energy to move automobiles. You can use any measurement you want — joules, watt-hours or the old-fashioned British Thermal Unit — but the more energy you can carry with you, the farther you can travel before stopping.

In more practical terms, and I apologize for getting all geeky on you, it’s really energy density — a measure of how much energy you can carry conveniently — that matters. More specifically, you want to package your onboard fuel as densely (energy per pound) and compactly (energy per litre) as possible, the first so that the car doesn’t have to haul around fuel-sapping extra weight, and the second so that more of the car’s volume can be devoted to passengers and their cargo and less to its fuel tank.

This, folks, is where the current state of battery technology fails so miserably. Gasoline’s energy density is amazingly potent. There are about 13,500 watt-hours (the ability to produce 13,500 watts for one hour or, conversely, one watt for 13,500 hours) of energy per kilogram of gasoline. Or, if you still think metric is the work of despots, just more than 6,000 watt-hours per pound.

On the other hand, the most optimistic numbers I’ve seen for lithiumion batteries is 250 watt-hours per kg or 110 watt-hours per pound. Yes, those numbers are right. No, I have not forgotten a few digits. That means good old-fashioned gasoline punches 54 times harder for the same amount of weight, the fundamental reason electric cars’ ranges are so pitiful compared with those fossil fuelled.

In the electric vehicles’ defence, electric motors transmit that energy more efficiently to the road. Some electric motors boast 90% efficiency, while internal-combustion engines can transmit as little as 15% of their energy into vehicular motivation. However, even being generous, that means EVs face a nine-times deficit versus traditional cars.

Even that understates the case. In fact, those 250 watt-hours I quoted earlier for lithium-ion most probably is the rating for just the tiny battery cells themselves–most battery packs used in electric cars are made up of small individual cells in foil packing stored in massive plastic arrays with complex electronics and, sometimes, cooling systems. For instance, the Mercedes SLS AMG E-Cell I tested has a battery rated at 48 kilowatt-hours. It weighs 450 kilograms. Even ignoring the fact Mercedes won’t let it discharge to less than 10% of its maximum or recharge to more than 95% (in the name of longevity), that’s a lowly 106 watt-hours per kg. I’ll let you do the long division as to the exact deficit that represents compared with gasoline.

Easier to understand is the comparison between the gasoline-powered SLS and the identical (except for the powertrain) E-Cell version — which is the first time we have had such a direct comparison between an EV and its gas-fed counterpart. The gas SLS, among the most profligate of internal-combusting automobiles, is rated at 10.3 litres per 100 km on the highway. With its onboard 85-litre gas tank, it could — assuming your right foot was more virtuous than Moses– travel more than 800 km before running out of fuel. By the way, said gas tank weighs about 60 kg when full.

The E-Cell version, on the other hand, has a claimed range of 150 km, although I managed only 120. Again, even being generous, the EV version has to carry seven times as much fuel onboard to go one-sixth the distance. These are not numbers that are going to reverse overnight no matter how much research is done. And that’s not mentioning the fact that the gas SLS could be completely refuelled in less than five minutes; the E-Cell would probably take close to 24 hours on a typical home’s 110-volt circuit.

Indeed, this infatuation with EV vehicles’ range may be the wrong numbers to be discussing. This is only a scientific wild-assed guess on my part, but I think that future mainstream EV adoption may rest in decreasing the recharging time, not in increasing range.

I can only speak for myself, but I would much rather travel only 200 km and have to spend five minutes recharging than going farther but then having to wait 45 minutes for my vehicle’s errant electrons to be returned to their proper home.

This may prove more problematic for newer battery technologies such as lithium-air, which promises far greater energy density than lithiumion and the possibility of housing the necessary hundreds of kilowatt-hours required for a decent range. Recharging them with dispatch, however, might be a huge problem involving more volts and amps than you or I might be comfortable near. But then, that’s a diatribe for another day.